Termination activation energy olefins

In conclusion, on the basis of the two activation-energy determinations, the rate-limiting step of terminal hydroformylations is not the dissociation of the trisphosphine complex, but rather a subsequent association of a bisphosphine derivative with the 1-olefin reactant. 

Since the 1-olefin concentration-dependent hydroformylation in the presence of the above catalyst system has a slightly higher activation energy of about 22 kcal mol-1, it is proposed that the ratedetermining step of selective terminal 1-olefin hydroformylation may involve a transition state leading to the formation of a 1-alkyl bis-(trans-phosphine)rhodium carbonyl hydride complex rather than the dissociation of the trisphosphine complex. 

It was suggested that a basic rule of thumb can be applied to determine which termination reaction predominates in a typical homopolymerization. Thus, polymerizations of 1,1-disubstituted olefins are likely to terminate by disproportionation because of steric effects. Polymerization of other vinyl monomers, however, favor terminations by combination unless they contain particularly labile atoms for transferring. Higher activation energies are usually required for termination reactions by disproportionation. This means that terminations by combination should predominate at lower temperatures. 

In general, a polymerization process model consists of material balances (component rate equations), energy balances, and additional set of equations to calculate polymer properties (e.g., molecular weight moment equations). The kinetic equations for a typical linear addition polymerization process include initiation or catalytic site activation, chain propagation, chain termination, and chain transfer reactions. The typical reactions that occur in a homogeneous free radical polymerization of vinyl monomers and coordination polymerization of olefins are illustrated in Table 2. 

B3LYP calculatimis showed that the insertion processes, which can lead to both the linear propyl (IL) and branched isopropyl (IB) complexes, have nearly the same activation free energies (25.8 and 26.5 kJ/mol) and reaction free energies (—4.5 and —4.1 kJ/mol). These results indicated that the insertion process does not determine the regioselectivity, in contrast with the proposal by Grima et al. [54]. On the other hand, the rather low activation barriers and much less exergonic properties suggested that the insertion processes are reversible. This explains the observed isomerization between internal and terminal olefins in experiment reasonably. 

Copolymerization with a-olefins over a Phillips catalyst is a key method for controlling the density and microstmctures of the polyethylene products in industrial processes. Table 5 also listed the energy barriers for the primary 1,2-insertion of 1-butene and 1-hexene, and the subsequent chain transfer by p-H elimination for all the three kinds of Ti-modified models. The calculated energy barriers showed that Ti-modification could also promote the activity for ethylene copolymerization with a-olefins. The energy differences between comonomer insertion and chain transfer can lead to a conclusion on the effect of Ti-modification on the distribution of the inserted comonomers in polyethylene chains. As listed in Table 5, the difference between energy barriers for chain propagation and for chain transfer decreased for model sites 4g, 12g, and 15g. Therefore, it was reasonable to conclude that Ti-modified catalyst was likely to make low MW polyethylene with much less comonomer insertion because the inserted comonomer mainly led to a chain transfer reaction and left the inserted comonomer at the chain end. As a result, the increased chain termination by comonomer resulted in less SCBs in the low MW fraction and higher density of the polyethylene product for the Ti-modified Phillips catalyst. 

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